1. Field of the Invention
The present invention relates to a photodetection type coordinate input device attached to the front side of a display and adapted to detect a coordinate position with respect to the display and input the detected coordinate position to a computer. More particularly, the present invention pertains to a coordinate input device which is so designed that it is prevented from malfunctioning.
2. Description of the Related Art
Among various types of coordinate input devices designed to manually input a coordinate position to a computer, photodetection type coordinate input devices in which a coordinate position can be specified simply by pressing any position on the front side of a display with a finger or the like have attracted attention recently from the viewpoint of reliability and operability.
One example of heretofore known photodetection type coordinate input device is shown in FIGS. 3 to 5, in which: FIG. 3 is a perspective view showing one example in which a photodetection type coordinate input device is attached to the front side of a display; FIG. 4 is a fragmentary sectional view showing an essential part on the light-receiving element side of the coordinate input device; and FIG. 5 is a rear view showing the internal structure of the coordinate input device with its rear panel removed.
This coordinate input device is composed of the following major elements: a frame 1 shaped such as to have a substantially rectangular configuration and having an opening 2 in the center; a plurality of pairs of light-emitting elements 4 such as LEDs and light-receiving elements 5 such as phototransistors, which are respectively disposed on the opposing sides of a display 3 constituted by a CRT or the like on the reverse side of the frame 1, that is, on the side of the display 3 which is closer to the display screen 3a thereof; and an arithmetic unit 6 adapted to detect a position at which optical paths 11 are blocked from the pairs of light-emitting elements 4 and light-receiving elements 5 and to input the detected coordinate position to the host computer side. The light-emitting elements 4 and the light-receiving elements 5 are soldered at their terminals to boards 7 mounted on the frame 1 in such a manner that the light-emitting portion 4a of one light-emitting element 4 and the light-receiving portion 5a of the light-receiving element 5 which is paired with this light-emitting element 4 oppose each other.
In the opening 2 is provided an operation panel 10 constituted by a VDT (Video Display Terminal) filter for making the display screen 3a of the display 3 easy to see and for preventing the screen 3a from becoming stained and also protecting the screen 3a from adhesion of dust or the like. In front of the arrayed light-receiving elements 5 is disposed a light-shielding plate 8 provided with light-transmitting bores 8a each having a predetermined area and depth which are so set that one light-receiving element 5 can receive only the incident light emitted from the paired light-emitting element 4. In addition, an infrared filter 9 in the shape of a rectangular frame which serves as a visible light-removing filter is provided along the entire periphery of the opening 2 so that the filter 9 is disposed in front of the light-shielding plates 8 and the light-emitting and -receiving elements 4 and 5. In other words, the respective front sides of the arrayed light-receiving elements 5 which are disposed in a substantially L-shape and the respective front sides of the arrayed light-emitting elements 4 which are also disposed in a substantially L-shape oppose the outer wall surfaces, respectively, of the infrared filter 9 in the shape of a rectangular frame. Accordingly, invisible optical paths 11 are formed through the infrared filter 9 on the front side of the operation panel 10, that is, the side of the panel 10 which is remote from the display screen 3a.
In detection of a coordinate position, as shown in FIGS. 3 and 5, for example, a position on the operation panel 10 which corresponds to a position on the display screen 3a that is desired to be input is pressed with a finger 12. In consequence, among the optical paths 11 formed along the surface of the panel 10, the optical paths 11 which would otherwise pass through the position of the finger 12 are blocked by the finger 12. Accordingly, by sequentially turning ON the light-emitting elements 4 so as to scan over the coordinates, it is possible to detect a blocked optical path 11 from the corresponding light-receiving element 5 in each of the directions of x and y. The blocked optical paths 11 are identified by the arithmetic unit 6, and the corresponding coordinate position is output to the host computer (not shown).
The above-described photodetection type coordinate input device suffers, however, from the following disadvantage. Undesirable light particularly from a light-emitting element 4 which is located near a corner of the opening 2 may be reflected from the approximately central portion of a nearby inner wall surface of the infrared filter 9 to reach the light-receiving portion 5a of a light-receiving element 5. In consequence, there is a fear that, even when a position on the operation panel 10 near the peripheral edge of the opening 2 is pressed with a finger 12, the blockage of the optical paths 11 may not be detected, and this leads to malfunction. To overcome such problem, an arrangement has heretofore been proposed in which a projection is provided at an approximate center of each inner wall surface of the infrared filter 9 so as to prevent any reflected light from reaching the light-receiving portion 5a of a light-receiving element 5. In practice, however, such malfunction may be caused not only by the reflection of light from an inner wall surface of the infrared filter 9 but also by the arrival at the light-receiving portion 5a of the light which propagates through the filter section, that is, the inside of the infrared filter 9 or the inside of the operation panel 10. For this reason, the above-described conventional proposition is not a satisfactory malfunction preventing measure. In addition, when relatively strong ambient light from a light source such as the sun or an incandescent lamp enters the filter section, the incident light may propagate through the inside of the infrared filter 9 or the inside of the operation panel 10 to reach the light-receiving portion 5a, thus causing malfunction.